The realization of optical amplifiers that use optical fibers doped with erbium (EDFA; erbium-doped fiber amplifier) in recent years has made it possible to directly amplify light signals in the 1.55 .mu.m wavelength band without conversion into electrical signals; high-capacity, long-distance communication is thereby practicable in the field of optical communication. Meanwhile, communication using wavelength division multiplexing (WDM), in which light signals of different wavelengths are transmitted by a single optical fiber, is being performed to expand the communication capacity of optical communication, and the application of the abovementioned erbium-doped fiber amplifier to an optical communication system using this wavelength division multiplexing technique is anticipated to expand the communication capacity further and realize long-distance transmission by the wavelength division multiplex technique.
In cases where an optical amplifier is to be applied to an optical communication system that uses wavelength division multiplexing, it is important that the optical amplifier amplify signal light of different wavelengths in a batch. However, it is well known that the gain of an optical amplifier using an erbium-doped fiber (EDFA) is wavelength dependent and thus gain difference arises among the wavelength division multiplex signal channels when wavelength division multiplex signals are amplified with an EDFA. The signal-to-noise ratios will thus differ among the wavelength division multiplex signal channels when an EDFA is applied to a wavelength division multiplexed optical communication system. Especially in the case of an optical communication system (optical transmission system) formed by connecting a plurality of EDFA's in a cascading manner between optical fibers for optical signal transmission, the signal-to-noise ratio of a signal of a channel with a small gain will deteriorate excessively in comparison to the signal-to-noise ratios of other channels and such differences in gain among wavelength division multiplex signal channels for different wavelengths restrict the transmission distance of a wavelength division multiplex type optical transmission system.
In general, an EDFA provides gain in a wavelength range of approximately 40 nm from 1525 nm to 1565 nm. However, it is known that with an EDFA that is supplied with adequate excitation power, the gain at a wavelength near 1530 nm will be 6 dB to 12 dB greater than the gain at a wavelength near 1550 nm. The gain is also non-flat in the range, 1540 nm to 1560 nm, where there are wavelength-dependent gain slopes and ripples.
Thus, in order to eliminate the wavelength dependence of the gain of an optical amplifier that uses an erbium-doped fiber, the insertion of an optical filter in the optical amplifier for flattening the gain characteristics of the optical amplifier has been considered. A method for setting the loss spectrum of the optical filter to be inserted in an optical amplifier that uses an erbium-doped fiber (EDFA) is described in "1995 Optical Amplifiers and Their Applications in a publication dated Jun. 15, 1995, Davos Switzerland, 1995 Technical Digest Series Volume 18 Sponsored by the Optical Society of America in a presentation entitled EDFA gain equalization with fiber filter for WDM systems by Eric Saint Georges denoted as ThD5 in the table of contents" (Reference 1). According to Reference 1, the loss spectrum of the optical filter is set so as to flatten the ASE (amplified spontaneous emission) when the input signal light is not input into the EDFA.
However, the gain spectrum of an EDFA is not only dependent on the wavelength of the input signal but is also dependent on the power of the input signal light and the excitation power of the pump light source. For example, the results shown in FIGS. 12 and 13 were obtained when the present applicant investigated the effects of the input signal light power and the length of the erbium-doped fiber on the wavelength division multiplex amplification characteristics of an EDFA excited at a wavelength of 0.98 .mu.m and a power of 65 mW.
The results of measuring the relationship between the input light power and the output light power upon input of signal light of the four different wavelengths of 1533 nm, 1539.5 nm, 1549 nm, and 1557 nm into an optical amplifier formed with an erbium-doped fiber with a length of 5 m is shown in FIG. 12 and those for an optical amplifier formed with an erbium-doped fiber with a length of 7 m is shown in FIG. 13. In either case, the input signal light power was varied in the range -16 dBm -30 dBm for each signal light, and the output signal light power for each input signal light power was measured with an optical spectrum analyzer.
As is clear from these Figures, for both the optical amplifier with an erbium-doped fiber with a length of 5 m and that with a fiber with a length of 7 m, not only did the output signal light power for a given input signal light power differ with the wavelength of the input signal light but the output signal light power also depended on the level of the input signal light power, and the proportion of the gain of the optical amplifier which differs according to the wavelength of the signal light, in other words, the gain difference also differed according to the input signal light power.
To clarify these differences further, the maximum gain difference of the optical amplifier due to differences in input signal light power (the width of the dispersion of the amplifier gain which differs with the wavelength of input signal light) was determined for each of the optical amplifiers with an erbium-doped fiber of a length of 5 m and 7 m on the basis of the measurement results shown in FIGS. 12 and 13. The results that were obtained are shown in FIG. 14.
As is clear from FIG. 14, for an input signal light power for example of -26 dBm, which is typically used for optical communication by the wavelength division multiplex technique, the maximum gain difference of an optical amplifier with an erbium-doped fiber of 5 m length is approximately 6.5 dB and the maximum gain difference of an optical amplifier with an erbium-doped fiber of 7 m length is also greater than 6 dB. Besides these dependencies of the optical amplifier gain on the input signal light power and length of erbium-doped fiber, it is also known that the gain depends on other factors such as the power of the pump light source provided in the optical amplifier.
Thus the gain spectrum of an EDFA will differ for the case where input signal light is not input into the EDFA as indicated in Reference 1 and the case where the input signal light is actually input into the EDFA, and for example, when an input signal light is input into an EDFA, the power of the pump light source must be increased to flatten the gain spectrum. Thus although power correction of the pump light source is performed in addition to the insertion of an optical filter in the EDFA in Reference 1, there is a problem in that the amount of this correction cannot be estimated beforehand.
The present invention has been made in view of the above problems and the objects thereof and provides a method of manufacturing an optical filter to be used in an optical amplifier that can realize a wavelength division multiplex type optical transmission system that can perform long-distance transmission and provides an optical filter manufactured by said method and an optical amplifier equipped with said optical filter.